The rule is

which is the power rule. You pull down the exponent to place it as the coefficient. So that explains the 1/2 pull out front. Then we subtract 1 from the exponent.
The expression you wrote can be simplified or rewritten like this
![(d)/(dx)x^n = nx^(n-1)\\\\(d)/(dx)\left[x^(1/2)\right] = (1)/(2)x^{(1)/(2)-1}\\\\(d)/(dx)\left[x^(1/2)\right] = (1)/(2)x^{-(1)/(2)}\\\\(d)/(dx)\left[x^(1/2)\right] = (1)/(2)\frac{1}{x^{(1)/(2)}}\\\\(d)/(dx)\left[x^(1/2)\right] = \frac{1}{2x^{(1)/(2)}}\\\\(d)/(dx)\left[x^(1/2)\right] = (1)/(2√(x))}\\\\](https://img.qammunity.org/2022/formulas/mathematics/high-school/pa84tf1tj3zwzrqnix9y6z4culfzuyosuk.png)
Optionally, we can multiply top and bottom by
to rationalize the denominator.